Baroness Thatcher has been forced to pull out of a Downing Street party to mark her 85th birthday after being taken ill with flu, Downing Street said today.

The former prime minister had been due to attend a gathering of around 150 friends and former colleagues hosted by David Cameron to mark the milestone.

It will still go ahead at her insistence, No 10 said, with a fresh event to be put on when she has recovered.

A spokesman said: "Lady Thatcher is unable to attend tonight's birthday party at No 10 after being taken ill with flu at home earlier today.

"At Lady Thatcher's insistence, the gathering will take place as planned in her absence and the Prime Minister will attend.

"Guests, who include previous members of her Cabinet, colleagues and friends, will be invited to return to celebrate her 85th birthday at No 10 once she has recovered."

Lady Thatcher's planned return to Downing Street was announced by the Prime Minister at the Conservative Party conference last week, when he was cheered by delegates for hailing Lady Thatcher "the greatest peacetime prime minister of the 20th century".

He first invited her to Number 10, from where she governed Britain from 1979 to 1990, soon after he became Prime Minister earlier this year.

The one-time "Iron Lady", whose birthday was yesterday, also returned to see Mr Cameron's two Labour predecessors, Tony Blair and Gordon Brown.

On the advice of doctors, Lady Thatcher very rarely speaks in public now, but does still attend a number of high-profile functions.

In March 2008 the peer, who has previously suffered minor strokes, was taken ill during a dinner in Westminster and spent the night in hospital as a precaution.